I never used to pay attention to the speed rating (called “speed class” by the industry that governs such things) on memory cards, because it never seemed to matter in my shooting. I never ran into problems regardless of the speed class of the card. But then along came video capabilities in SLRs and much larger file sizes from the still images. Turns out it matters quite a bit these days.
One day I was shooting full-hd video (1080p) with my Canon 6D, and I kept getting the “Recording Was Suspended Automatically” error message. I also noticed that it was taking longer than usual for the camera to finish writing a short burst to the card. I was in a hurry to start shooting (kid was doing something cute), so I grabbed the first SD card I could find. It happened to be a very old card with a speed class rating of 4. This is nowhere near enough to handle 1080 video (see table below).
Once I figured out what was going on, I put a Class 10 card in the camera and all was well again. Class 10 works well for everything up to about 2K video. I have used Class 10 (also called U1 or V10) in my DJI Phantom 3 Standard without any issues (2.7K).
Why Does it Matter?
Digital cameras have a memory buffer where captured images are stored until they can be written to a memory card. The more expensive cameras will have larger buffers, but all of them have a limit. If the buffer gets full, you have to wait to take more pictures or capture video. Once the buffer offloads enough data, it can start taking in new photos or videos. The bottleneck in this process is usually the buffer. But if the card is older/slower, it can cause a bottleneck as well.
Upgrade Now if You Have Older Memory Cards
Even a budget-minded guy like me can’t put a price on memories. With a slow memory card, you could miss capturing a once-in-a-lifetime memory. So get some Class 10 or higher cards now to avoid future regret! Besides, memory cards are cheap. Don’t be so phrugal you miss out great photography.
What Speed Class Rating Do You Really Need?
The fine folks at sdcard.org put together a great chart for this. Rather than reinvent the wheel, I’ve borrowed their chart and posted here. Click the chart to read a great description of the various speed rating levels.